Method for forming an electric insulating film having excellent punching property and heat resistance on an electrical steel sheet

ABSTRACT

A method for forming on an electrical steel sheet an electric insulating film having excellent punching property and heat resistance comprising coating on the steel sheet a treating solution comprising a composition composed of one or more of chromic anhydride, chromate, bichromate and 1 - 35 parts (as solid) of styrene resin containing not less than 50% stylene and/or vinyl acetate resin emulsion per 10 parts of chromium ion in the composition and heating the coating at a temperature between 150* and 500*C.

United States Patent Kato et al. Sept. 30, 1975 [54] METHOD FOR FORMING AN ELECTRIC 3,477,881 1 H1969 Wacla et a1 148/62.

INSULATNG FILM HAVING EXCELLENT gavlik 1rs e a PUNCHING PROPERTY AND HEAT 3,649,372 3/1972 lmai et 148/616 RESISTANCE ON AN ELECTRICAL STEEL 3,785,882 1/1974 Jackson 148/113 SHEET 3,793,073 2/1974 Kitayama et al .4 1 17/230 Inventors: l-lirotada Kato; Kazuo Nakamura,

both of Kitakyushu, Japan Assignee: Nippon Steel Corporation, Japan Filed: Sept. 21, 1973 Appl. No.: 399,625

Foreign Application Priority Data Sept. 29, 1972 Japan 47-97777 u.s. c1. 148/113; 117/230; 117/234; 148/315; 148/62 1111. C1. ..1101F 1/04 Field 61 Search 148/113, 6.2, 6.16, 31.5, 148/122, 6.15; 117/230, 234

References Cited UNlTED STATES PATENTS 6/1968 Kohler 148/113 Primary ExaminerWalter R. Satterfield Attorney, Agent, or Firm-Toren, McGeady and Stanger ABSTRACT 5 Claims, 2 Drawing Figures U.S. Patent Sept. 30,1975 3,909,313

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ylene resin solid mponenl pan ArlOparls '2 4 e s |ol2 l4 l5 l6l8 2 o2z 4 Number of punchings up to the burr allowance limit of 50 1 lxlO limes) FlG.2

Example2 b Cl Com parafive Example 3 C (only A solution) so i Burr height 91) 2 4 6 8 l0 l2 l4 I5 l6 I8 20 22 24 Number of punchings (XlOlimes) METHOD FOR FORMING AN ELECTRIC INSULATING FILM HAVING EXCELLENT PUNCHING PROPERTY AND HEAT RESISTANCE ON AN ELECTRICAL STEEL SHEET The present invention relates to a method for forming an electric insulating film having excellent punching property and heat resistance on the surface of an electrical steel sheet. The'electrical steel sheet used herein includes all of so called silicon steels and low carbon steels usable for iron cores of electrical appliances.

It is well known that silicon steel sheets or low carbon iron plates in which impurities, such as, carbon is reduced as low as possible have been used for electrical instruments, such as, transformers and generators.

The electric insulating filmcoated on'the electrical steel sheets is required to have excellent inter-layer resistance, film adhesion, corrosion resistance, weldability and punching properties in addition to electric insulating properties.

The punching property of the electrical steel sheet has an important influence on the dies for punching the sheet, and excellent punching property assures increased number of punchings per onedie and thus considerably elongates the life of the die.

The punching property can be improved to some degree by material improvement of the electrical sheet itself, but there is a limitation to the degree of improvement that can be effected by this alone.

The present invention will be described in detail by referring to the attached drawings.

FIG. 1 is a graph showing the relation between the additional amount of styrene resin and the punching property, and

FIG. 2 is a graph showing the relation between the burr height and number of punchings in the comparison I and Examples 2 and 3.

The present inventors have found that the punching property is largely influenced by the electric film formed on the surface of the electrical steel sheet and have conducted various extensive studies on the basis of the discovery, and have succeeded in developing film agents which make possible to increase the number of the punchings per one die to 2,500,000 that is, 5 to 30 times as compared with the conventional insulating film, and a method for forming the film.

The conventional film agent, for example, an insulating film formed with a treating solution prepared by mixing divalent metal oxides with chromic acid anhydride shows the punching property as shown by the curve (a) in FIG. 2, where with about80,000 punchings, burrs reach 50p. of the allowable limit.

Further, heat resistance is one of the important properties of the electric insulating film. Generally, heat treatment of the electrical steel sheets is done in the piled form in order to remove the working strain and improve magnetic characteristics, and is usually done in a neutral or weak-reducing atmosphere, depending on the purposes of the treatment. In this case, heat resistance, namely, non-peeling of the insulating film is required.

Further, since generators for refrigerator compressors use freon gas if on the exposed edge surface of the iron core made by punching the electrical steel sheet, rust is formed and the rust strips off, it can get into the gas and clog the expansion valve. Further the exposed metal serves as a catalyst and decomposes the freon gas. In order to solve such problems, the punched edge surface is subjected to a blueing treatment. In this treatment also, it is required that the insulating film not peel off; powdering does not occur at the treating temperature between 400 and 600C, and the piled sheets do not adhere to each other. Thus an additional type of heat resistance is required.

Therefore one purpose of the present invention is to provide an electric insulating film for an electrical steel sheet, which film does not undergo thermal deterioration which cause the above troubles during the heat treatments, and which also possesses remarkably improved punching properties.

The steel plate sheet or strip to which the present insulating film is applied include silicon steel plates or low carbon iron plates containing very small amounts of silicon or containing no siliconwhich are generally used for electrical appliances, such as, generators and transformers.

As the insulating film agents for the electrical steel plate, a mixture of chromate and resin emulsion is known as disclosed in Japanese patent publication Sho 40-6722.

In general, when the treating solution ofchromic acid with the addition of resin emulsion is coated and baked on the steel plate, the agent serves as a good solid lubricant and gives good punching property because of the small emulsion particles (0.1 to la).

However, when most of the above resin emulsions are added to the chromic acid solution, the resin particle is broken and instantly gells or gradually gells and produces sediment.

Further, when the above treating solution is applied on the steel plate by rolls, the sediment adheres to the rolls and thus a uniform film can not be obtained.

Meanwhile, when commercially available emulsion resins, such as, vinyl acetate emulsion using PVA protection colloids for paints, acrylic acid ester emulsion, or ethylene vinyl acetate emulsion are added to the chromic acid solution and the thus prepared treating solution is applied on the electrical steel plate, only poor heat resistance is obtained and the film peels off or powders during the heat treatment.

Therefore, another object of the present invention is to provide a method for formingan electric insulating film which has overcome the above defects. The treating solution for forming the film according to the present invention may be prepared by adding styrene resin to a composition (A) containing one or more of chromic anhydride, chromate and bichromate.

As for the chromate or bichromate of the above com position (A), salts of bivalent metals, such as, magnesium, calcium and zinc, may be used.

Also, an aqueous solution prepared by dissolving oxides of bivalent metals, such as, MgO, CaO, ZnO; hydroxides, such as, Mg(OH) Ca(OI-I) and Zn(OH) or carbonates,'such as, MgCO CaCO and ZnCO in chromic anhydride in an amount less than their dissolution limit may be used. This aqueous solution composition may be considered to be an aqueous solution of a mixture of chromic anhydride and chromate. When the above oxide, hydroxide or carbonate of the bivalent metal ion is added to the dissolution limit, the heat resistance of the film lowers, and the film peels off during the strain-relief annealing step and the blueing step.

Further, when these additions are added in an amount more than their dissolution limit, non-dissolved powders are applied together with the treating solution so that the surface luster is lost and the film easily peels off near the edge surface during the stamping of cores.

As for the styrene resin to be added to the composition (A), styrene emulsion resin, styrene-acryl copolymer emulsion, and styrene vinyl acetate resin may be used.

In case of a copolymer of the styrene resin and other resins, it is required that styrene'is' contained in an amount of 50% or more as solid component. If the proportion of styrene is less than 50%, the heat resistance is poor so that the film will peel offor powder during the heat treatment. a

On the other hand, a mixture of styrene emulsion and emulsion of other resins also can give satisfactory heat resistance.

The proportion of the styrene emulsion to the composition (A), for example, to chromic anhydride, is l to 35 parts to parts of chromium ions contained in the chromic anhydride. With less than one part addition of the styrene resin, no effect is obtained by the addition and the desired punching property can not be obtained. On the other hand, if the addition is over 35 parts, the punching property is not improved any further, but the film softens during the heating and is easily damaged.

Further, poor conductor-forming oxides, such as, boric acid, colloidal silica, colloidal alumina, sodium silicate and potassium silicate may be added to the treating solution used in the present invention without deviating fromthe scope of the present invention.

The above oxides, however, should be added in a range from 2 to 16 parts to 10 parts of the chromium ions in the composition (A).

The water-soluble saturated dicarboxylic acids having four or more carbon atoms used in the present invention are acids, such as, succinic acid, adipic acid, glutamic acid, and pimelic acid. Forthe reduction of hexavalent chrome, the above carboxylic acid is used, and when other carboxilic acids are used, the following defects arise.

For example, with mono-carboxylic acids, such as, formic acid and acetic acid, the reducing force is so large that the reduction is effected in such a degree as to precipitate the inorganic substances and of the emulsion resin, thus damaging the stability of the solution, and result in a film having poor appearance,adhesion and heat resistance. Further, when saturated carboxylic acids such as, oxalic acid having two carbon atoms are used, agood film is not obtained, and with the use of nonsaturated carboxylic acids, such as, maleic acid, the stability of the solution is poor. Only when saturated carboxylic acids having 4 or more carbon atoms are used is the stability of the inorganic substances and the emulsion very excellent and a good film for an electrical steel sheet can be obtained.

, Polyethylene glycols having about 200 to about 1000 molecular weight are most preferable for use in the present invention. Ethylene glycol has so strong a reducing force that the solution stability is not obtained, and similarly polyhydric alcohols, such as, glycerine, amines, such as, triethanolamine, are not usable as they promote settlements of inorganic substances and of the emulsion resins.

The optimum proportion of the water-soluble saturated carboxylic acid having 4 or more carbon atoms and the polyethylene glycol is from 1 to 10 parts to 10 parts of the chromium ion. With less than 1 part, the reducing force is low and a large amount of hexavalent chromium remains in the film and the film dissolves in water. Even with more than 10 parts, the effects of the reducing agent will not change substantially.

The treating solution having the above composition may be applied on an electrical steel plate having suitable surface conditions by any of the conventional coating methods, such as, roll coating, brushing, spraying and immersion.

Although there is no special limitation in the coating amount of the treating solution, it is most. desirable that the residual solid substance after the heating and baking is 0.5 to 5 g/m According to the present invention, the baking after the coating of the treating solution is done at a steel surface temperature of to 500C and the heating time for the baking is not specifically limited, but a heating time between 10 and 180 seconds will give good results.

When the heating temperature is lower than 150C, no satisfactory baking is obtained and the film has no water-resistance and becomes viscous by absorption of water. On the other hand, when the heating temperature is higher than 500C, the film agent is decomposed and thus the film adhesion and the punching property become poor thus failing to obtain the desired results of the present invention.

The electric insulating film formed by the method of the present invention is excellent in both of the punching property and heat resistance, and the solution itself has good stability and can stand for long time.

The styrene emulsion resin is prepared by polymerization under the presence of an emulsifying agent (mainly a surface activating agent), and the stability of the emulsion resin in the treating solution of the present invention is influenced by the kind of the emulsifying agent to used. When an anionic activating agent, for example, laulylic sodium sulfate is used, the emulsion resin readily settles in a short time. Nonion activating agents (for example, polyoxyethylene nonyl phenyl ether, polyoxyethylene alkyl-aryl ether) or nonionanion mixture activating agents (for example, a mixture of polyoxyethylene noel phenol ether and dodecylbenzene sulfoninate) are preferable. I

By using the above specifiedreducing agents (saturated carboxylic acids having 4 or more carbon atoms and polyethylene glycol) and stylene emulsion resins polymerized under the presence of nonion activating agents or nonion-anion mixture activating agents, the settlement and gellation of the emulsions is effectively prevented and the emulsions can be preserved for more than one month.

The vinyl acetate emulsions used in the present invention should be polymerized using nonion activating agents or nonion-anion mixture activating agents, and the vinyl acetate emulsions which are polymerized using high-moelcular protection colloids, such as, commercially available PVA and cellulose derivatives for paints, should be avoided. When the latter emulsions are used, not only is the solution stability is poor, but

- also thermal deterioration is observed during the heat Therefore, the vinyl acetate emulsion resins in the EX M E 4 present invention means the vinyl acetate emulsion res- A treating solution having the following composmon: ins and the vinyl acetate-acryl copolymerizatlon emul- C I h t 75 K a ClUm C I'Omfl 8 sion prepared by using the above activating agents. Chromic h d d 12 The present invention will be more clear from the fol- 5 Boric acid 6 Kg 1 1 Succinic acid 4.5 Kg

Owlng examp Styrene acryl copolymerization emulsion EXAMPLE 1 (resin solid 40%, ratio of stylene to butylacrylate 9 l by weight) Treating solutions prepared by adding 2 parts, 5 Water 400 1 parts 15 P 20 parts and 30 parts 9 styrene'acryl was left at ordinary temperatures for 1 month and then COPOIYmeHZaUOP (total Sohd Components coated by rolls on an electrical steel sheet, and heated acryl 10%1 the 50nd components) to the compo at a furnace temperature of 500C for 35 seconds, smon A comprising 20 parts of chroml c anhydrld thereby a uniform and lustrous film was formed on the (chrome 10 parts)* 6 Parts of magnes'um oxlde, 2 steel sheet in an amount of 2.3 g/m The solution was parts of adipic acid (reducing agent) and 250 parts of Stable and no settlement was Observed water were coated by rolls uniformly on an electrical steel sheet, heat-treated at a sheet temperature 350C EXAMPLE 5 7 g a umform msulatmg film m an amount of 2 A treating solution having the following composition:

rn g Chromic anhydride 20 Kg The characteristics and punching property of the film Magnesium oxide 5 Kg are shown in Table l. The stability of the above treating sBoric acid 5 Kg uccinic acid 2 Kg solutions were very excellent and no settlement was ob- Polyethylene glycol (molecular served after more than one month. weight 400 2 Kg 40% polystylene emulsion resin 10 Kg EXAMPLE 2 40% vinyl acetate emulsion l0 Kg Water 300 Kg A treating solution having the following composition: was left at Ordinary temperatures for 1 month and then Chromic anhydride 20 parts coated by rolls on an electrical steel sheet and heated g f-g 2 Paris at a. sheet temperature of 300C, thereby a uniform and one act parts I Exhylene glycol (reducing agent) 2 pans 3O lustrous film was formed on the sheet in an amount of Styrene emulsion (solid component) 2.1 g/m NO gellation Of the solution and no settlement 10 parts water 300 parts were obseryed. The punching property test showed 1,200,000 times and the adhesion property test showed was coated by rolls on a silicon steel sheet, heated at a less than 10mm results of the reslstance o test and the heat resistance test were similar as those of sheet temperature of 280 C to form a film in an amount the other exam leg of 2.6 g/m whose characteristics are shown in Table 1. EXAMPLE 6 m EXAMPLE 3 A treating solution having the following composition:

A treating solution having the following composition: 40 Chromic anhydride 20 Kg Magnesium oxide 6 Kg Chromic anhydride 25 Kg 9 4 Kg Succmrc acid 4 Kg oxde 3 fi 40% vinyl acetate acryl copolymerization 1 g emulsion 30 Kg 2x 325 231 gsg zgg g g i L8 Kg (ratio of vinyl acetate to butyl a'crylate 7 3) l 35 K ater 300 Kg who of Style: m was left at ordinary temperatures for'one month and Water 4001 then heated at a sheet temperature of 300C, thereby a uniform and lustrous film was formed on the sheet in was left at ordinary temperatures for 15 days and then an amount of 2.1 g/m No gellation of the solution and coated by rolls on an electrical steel sheet and baked no settlement were observed. The punching property at a sheet temperature of 300C, thereby a lustrous and test showed 1,600,000 times and the adhesion property uniform film'was formed on the steel sheet in an test showed less than 10 mm. The corrosion resistance amount of 1.8 g/m No gellation of the solution and no and the heat resistance test were similar as those in settlement were observed. other examples.

" Table 1.

Punching property and film characteristics V Punching Ex. Treating Solution property *1 Adhesion No. (in thousand) Property *2 l A Solution Styrene Resin parts 2.5 200 20 5 600 10 10 1200 less than 10 15 1600 20 2050 35 2400 2 1350 3 2300 4 1800 Table 1 -Continued Punching property and film Characteristics Solution (2) v Punching Ex. Treating Solution property *1 Adhesion No. (in thousand) Property *2 Com- P rative l A Solution 80 20 d) 2 A Solution Vinyl Acetate Resin *3 l5 parts 1400 30 d 3 A Solution Acryl Resin parts I050 4) Heat Resistance *5 Corrosion Ex. Resistance Stress-relief Annealing Blueing No. *4 Cloth Vinyl Vinyl tape tape l Good Good No peeling No peeling I, u H H H H H H I, H H ,1

H n H H 2 I! I! I I! I! 3 I, I, H n 4 H I, I, ,1 Com- P rative Slightly l bad H I I, 2 Bad Complete Complete peeling peeling 3 Good Bad Partial Partial peeling peeling *l Punching property: Number of punchings up to the burr allowance limit of 50p '2 Adhesion property: Diameter of the smallest arc which peels off when the sheet is bent in a predetermined arc (180) and bent hack. "3 Commercially available polyvinyl alcohol protection colloids are used.

was attached to the surface and peeled off The comparisons in Table 1 show the results obtained by coating the following solutions (1 (2) (3) by rolls on the steel sheet and heating at 300C to form a film in an amount of 2 g/m Solution (1) only the composition A the composition A 15 parts as solid of vinyl acetate emulsion (solid 50%) the composition A l0 parts as solid acrylic acid ester emulsion (solid 40%) Solution (3) cellent in the present invention.

What is claimed is:

lected from the group consisting of saturated dicarboxylic acids having 4 or more carbon atoms and polyethylene glycol; and l to 35 parts of an aqueous emulsion of a styrene resin containing at least on a solids basis of styrene or an aqueous vinyl acetate emulsion prepared using a nonionic or mixture of a nonio'nic and anionic activating agent, the vinyl acetate emulsion being prepared in the absence of a high molecular weight protective colloid; and then baking the sheet at a temperature of 150 to 500C. j

2. A method for forming an electric insulating film as claimed in claim 1, wherein the saturated dicarboxylic acid is adipic acid, succinic acid, pimelic acid or glutaric acid.

3. A method for forming an electric insulating film as claimed in claim 1, wherein the polyethylene glycol has a molecular weight from 200 to 1000.

4. A' method for forming an electric insulating film as claimed in claim 1, wherein the'aqueous'styrene resin 1. A method for forming an insulating film having exmetal or a bichromate of a bivalent metal; 1 to 10 parts per ten parts on a weight basis of a reducing agent seemulsion containing 50% or more of styrene is a styrene emulsion, a styrene-acrylic copolymer emulsion or a styrene-vinyl acetate emulsion.

5. A method for forming an electric insulating film as claimed in claim 1, wherein the aqueous vinyl acetate emulsion is a vinyl acetate emulsion or a vinyl acetateacryl copolymer emulsion. 

1. A METHOD FOR FORMING AN INSULATING FILM HAVING EXCELLENT PUNCHING AND HEAT RESISTANT PROPERTIES ON THE SURFACE OF AN ELECTRICAL STEEL SHEET COMPRISING APPLYING TO AN ELECTRICAL SHEET, A COMPOSITION CONSISTING ESSENTIALLY OF CHROMIC ANHYDRIDE, A CHROMATE OF A BIVALENT METAL OR A BICHROMATE OF A BIVALENT METAL: 1 TO 10 PARTS PER TEN PARTS ON A WEIGHT BASIS OF A REDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF SATURATED DICARBOXYLIC ACIDS HAVING 4 OR MORE CARBON ATOMS AND POLYETHYLENE GLCOL, AND 1 TO 35 PARTS OF AN AQUEOUS EMULSION OF A STYRENE RESIN CONTAINING AT LEAST 50% ON A SOLIDS BASIS OF STYRENE OR AN AQUEOUS VINYL ACETATE EMULSION PREPARED USING A NOIONIC OR MIXTURE OF A NONIONIC AND ANIONIC ACTIVATING AGENT, THE VINYL ACETATE EMULSION BEING PREPARED IN THE ABSENCE OF A HIGH MOLECULAR WEIGHT PROTECTIVE COLLOID, AND THEN BAKING THE SHEET AT A TEMPERATURE OF 150* TO 500$C.
 2. A method for forming an electric insulating film as claimed in claim 1, wherein the saturated dicarboxylic acid is adipic acid, sUccinic acid, pimelic acid or glutaric acid.
 3. A method for forming an electric insulating film as claimed in claim 1, wherein the polyethylene glycol has a molecular weight from 200 to
 1000. 4. A method for forming an electric insulating film as claimed in claim 1, wherein the aqueous styrene resin emulsion containing 50% or more of styrene is a styrene emulsion, a styrene-acrylic copolymer emulsion or a styrene-vinyl acetate emulsion.
 5. A method for forming an electric insulating film as claimed in claim 1, wherein the aqueous vinyl acetate emulsion is a vinyl acetate emulsion or a vinyl acetate-acryl copolymer emulsion. 